Abstract Pain is a symptom of numerous clinical disorders that afflicts millions of people worldwide. Systemic administration of opioid analgesics remains the most effective treatment for many severe pain conditions. This class of analgesics is often associated with the development of side effects including hyperalgesia, tolerance and dependence, all of which severely limit their effectiveness in pain control. A promising strategy to combat these problems is to develop adjuvants of opioids aimed at increasing opioid efficacy and reducing opioid untoward effects. The endogenous neuropeptide FF (NPFF), a member of the RF-amide family of peptides, interacts with two distinct G protein-coupled receptors, NPFFR1 and NPFFR2. Emerging evidence indicates that NPFFR1 antagonism attenuates hyperalgesia and tolerance to opioids, two major side effects which hinder the use of opioids in pain management. Most of reported NPFFR ligands are peptides or peptidomimetics with a guanidine functional group which often has poor CNS penetration, thus not suitable for systemic administration. Recognizing the unmet need to develop potent and drug-like small molecule NPFFR1 antagonists, we have conducted a high throughput screening campaign and identified a novel proline derivative as a promising hit. Preliminary structure-activity relationship studies have improved the potencies of these compounds. In this application, we propose to further optimize this proline scaffold to improve potency, selectivity and drug-likeness.